(57) Peptides which inhibit the secretion of gonadotropins by the pituitary gland and
inhibit the release of steroids by the gonads. Administration of an effective amount
prevents ovulation of female mammalian eggs and/or the release of steroids by the
gonads. The peptides have the structure: X-R1-R2-D-Trp-Ser-R5-R6-Leu-Arg-Pro-R1 wherein X is hydrogen or an acyl group having 7 or less carbon atoms; R, is dehydro-Pro,
D-pGlu, D-Phe, D-Trp or β-D-NAL; R2 is Cl-D-Phe, F-D-Phe, CαMe-4-Cl--D-Phe, NO2-D-Phe, Cl2-D-Phe or Br-D-Phe; Rs is Tyr, l-Tyr, CH3-Phe, F-Phe or Cl-Phe; R6 is a D-isomer of a lipophilic amino acid or is 4-NH2-D-Phe, 4-gua-D-Phe, D-His, D-Lys, D-Orn, D-Har or D-Arg; and R10 is Gyl-NH2, D-Ala-NH2 or NH-Y, with Y being lower alkyl, cycloalkyl, fluoro lower alkyl or
where Q is H or lower alkyl. When R, is β-D-NAL, R6 is 4-NH2-D-Phe, 4-gua-D-Phe, D-His, D-Lys, D-Orn, D-Har or D-Arg. When R5 is Tyr or 2-Cl-Phe, either Orn, AAL or aBu is substituted for Ser, or R, is β-D-NAL
and R6 is D-His or 4-gua-D-Phe. NαMe-Leu may optionally be substituted for Leu.
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